1. Field of the Invention
The present invention relates to a sheet conveying device and an image forming apparatus that includes the sheet conveying device.
2. Description of the Related Art
In recent years, in color image forming apparatuses, an intermediate transfer method that primarily transfers a toner image on a photosensitive body on an intermediate transfer body, and then secondarily transfers the toner image in four colors on the intermediate transfer body on a sheet has been widely used. The image forming apparatus using the intermediate transfer body has an advantage of high versatility that various types of sheets such as a thin paper, a thick paper, a postcard, and an envelope can be used. An intermediate transfer drum or an intermediate transfer belt is generally used for the intermediate transfer body.
However, when a sheet above a certain thickness enters a secondary transferring unit, the speed of the intermediate transfer body driven at a constant speed up to then fluctuates for a short time, thereby causing a problem of generating a distortion in an image at a primary transferring unit.
With miniaturization of color image forming apparatuses, the secondary transferring unit and a fixing unit have been arranged close to each other, and in some apparatuses, the transferring and the fixing of an image are carried out on the sheet at the same time. In such an apparatus, when the sheet above a certain thickness enters the fixing unit, the speed of a fixing roller or a fixing belt that were driven at a constant speed up to then also fluctuates for a short time, thereby causing the same problem of generating a distortion in an image at the secondary transferring unit.
These problems can be prevented by feedforward control that, before the sheet enters the secondary transferring unit or the fixing unit, estimates the entry timing, and negates a speed fluctuation by increasing the speed and a torque of the intermediate transfer body and the like that fluctuate at the entry of the sheet. In related art, the following proposals have been made.
Japanese Patent Laid-open Publication No. 2003-215870 discloses an image forming apparatus that measures the time from the start of printing to the entry of a sheet into a transferring unit, and uses the measured time originating the start of printing as the timing for the next feedforward control.
Japanese Patent Laid-open Publication No. 2005-107118 discloses an image forming apparatus that measures the time from the start of clutching of a resist roller to the entry of a sheet into a secondary transferring unit in advance, and uses the measured time originating the start of clutching of the resist roller as the timing for the next feedforward control.
Japanese Patent Laid-open Publication No. 2004-54120 discloses an image forming apparatus installed with a paper detection sensor immediately before a fixing unit, and carries out the feedforward control by the detection signal.
In this manner, in the related-art image forming apparatus, the entry timing (timing of the entry of a sheet into the transferring unit and the fixing unit) to carry out the feedforward control is triggered by various forms such as the start of printing, the start of clutching of the resist roller, or the detection by the paper detection sensor. However, the following problems occur:
In the image forming apparatus disclosed in the Japanese Patent Laid-open Publication No. 2003-215870, a fluctuation occurs to the time from the start of printing to the entry of the sheet into the transferring unit, thereby causing an error to the measured time obtained in advance. Accordingly, it is difficult to obtain the accurate timing for the feedforward control.
In the image forming apparatus disclosed in the Japanese Patent Laid-open Publication No. 2005-107118, similarly to the above, a large fluctuation occurs to the time from the start of clutching of the resist roller to the entry of the sheet into the secondary transferring unit, thereby causing an error to the measured time obtained in advance. Accordingly, it is difficult to obtain the accurate timing for the feedforward control. Even with the steady clutching time, in practice, the entry timing of the sheet fluctuates every time, thereby making it difficult to carry out the accurate feedforward control using the value measured in advance, and causing a problem in the accuracy. Because the entry timing of the sheet changes due to deterioration of parts and deterioration with age, it is difficult to secure stability over time.
In the image forming apparatus disclosed in the Japanese Patent Laid-open Publication No. 2004-54120, the paper detection sensor is installed immediately before the fixing unit, and the feedforward control is carried out by the detection signal. However, the timing from the detection of the sheet by the sensor to the entry of the sheet into the fixing unit fluctuates due to the error in the sensor detection position, thereby making it difficult to obtain the accurate timing for the feedforward control. Moreover, there was a limitation in reducing the distance between the detection sensor and the fixing unit to reduce the detection error, due to the structural restriction.